1. Field of the Invention
The present invention relates generally to a complementary metal oxide semiconductor (CMOS) image sensor and a method of fabricating the same. More particularly, the invention relates to a CMOS image sensor providing uniform pixel exposure by avoiding overflow of electrical charges from a photodiode region formed therein and a method of fabricating the same.
A claim of priority is made to Korean Patent Application 2004-94704 filed on Nov. 18, 2004, the disclosure of which is hereby incorporated by reference in its entirety.
2. Description of the Related Art
A wide array of consumer and industrial electronic devices incorporate image sensors. These devices include, for example, digital cameras, cellular phones, web cameras, personal digital assistants, and digital video cameras, to name but a few.
An image sensor converts light into electrical signals, which are then used to form an image. The light is generally supplied to the image sensor through a collection of optical components such as, for example, a lens. The electrical signals are then processed by a collection of electronic components including, for example, an amplifier, an analog to digital converter (ADC), etc., to produce the image.
The image typically comprises an array of pixels arranged in a matrix and each pixel generally comprises a set of discrete data values representing intensities and/or colors of light received by the image sensor.
Each pixel is generated by a corresponding pixel sensor located in a pixel sensor array in the image sensor. Each pixel sensor comprises a light detection unit (e.g., a photodetector), a transmission unit, and a signal output unit.
Image sensors may be broadly categorized in two classes, namely charge coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors. Both classes of image sensors are widely used and both have advantages over the other.
A CCD image sensor accumulates electrical charges in the photodetector of each pixel sensor and then transfers the electrical charges from each pixel sensor to a common output structure. The common output structure converts the electrical charges to voltages, buffers the voltages, and transfers the voltages to other electrical elements for processing.
In contrast to the CCD image sensor, a CMOS image sensor accumulates electrical charges in the photodetector of each pixel sensor, and then locally converts the electrical charges into voltages at each pixel sensor. Accordingly, each pixel sensor in a CMOS image sensor typically contains additional circuitry such as a buffer, an amplifier, and so forth. This difference has significant implications for CMOS image sensor architecture as well as capabilities and limitations of the CMOS image sensor.
Some advantages of using a CCD image sensor instead of a CMOS image sensor include a superior ability to arbitrarily start and stop exposure, i.e., electronic shuttering, a superior dynamic range, more uniform exposure of pixels under identical lighting conditions, and a lower susceptibility to noise caused by on chip circuitry.
On the other hand, some advantages of using a CMOS image sensor instead of the CCD image sensor include the fact that CMOS image sensors can be fabricated using general CMOS processing techniques. Using general CMOS processing techniques to fabricate the CMOS image sensor allows various electrical elements to be readily formed near each pixel sensor in the pixel sensor array. The electrical elements may include, for example, amplifiers, filters, buffers, and so forth. The electrical elements formed near each pixel sensor allow pixels to be transferred and/or processed by the pixel sensor much faster than in a CCD image sensor. Accordingly, higher frame rates can generally be obtained by CMOS image sensors. In addition, in CMOS image sensors, small regions of an image can be processed independent of other regions since each pixel sensor has the ability to locally convert electrical charges into voltages.
Another advantage of the CMOS image sensor is that it is more power efficient than the CCD image sensor. The CCD image sensor consumes a large amount of power because it uses an external control signal and large clock swing operations to control the transfer electrical charges.
One significant problem with CMOS image sensors, however, is that pixels tend to be non-uniformly exposed under identical lighting conditions. This is due, at least in part, to the architecture used in most CMOS image sensors and the way in which they operate. This is described in relation to FIGS. 1A, 1B and 2.
FIG. 1A is a diagram illustrating a pixel sensor in a conventional CMOS image sensor (CIS) and FIG. 1B is a diagram illustrating an overflow of electrons from one region of the pixel sensor to another during a read operation of the CIS.
Referring FIGS. 1A and 1B, a pixel sensor in a conventional CIS includes a photodiode region PD formed in a semiconductor substrate 10, a transfer gate (TG) 22, and a reset gate (RG) 23. A light shielding plate 30 is formed over transfer gate 22 and reset gate 23.
Incident light generates free electrical charges in photodiode region PD. Transfer gate 22 controls a transfer of the electrical charges from photodiode region PD to a floating diffusion (FD) region 12. Meanwhile, reset gate 23 controls a removal of the electrical charges from floating diffusion region 12. Floating diffusion region 12 is connected via a connection line 40 to the gate of a source follower transistor (not shown) which is adapted to detect a potential associated with the electrical charges accumulated in floating diffusion region 12.
Connection line 40 is formed by a method comprising forming an interlevel insulation film covering gates 22 and 23, patterning the interlevel insulation film to form a contact hole exposing floating diffusion region 12, and forming a contact plug to fill up the contact hole.
Unfortunately, floating diffusion region 12 is often damaged when the interlevel insulation film is patterned. Furthermore, the contact plug filling the contact hole often contaminates floating diffusion region 12 where the contact plug is formed of a metallic material. The damage and contamination caused to floating diffusion region 12 can lead to leakage current in the pixel sensor and white spots in pixels generated by the pixel sensor.
Because the conventional CIS is not equipped with a mechanical shutter, incident light is received by photodiode region PD while a read operation of the pixel sensor is performed. Accordingly, free electrical charges are continuously accumulated in photodiode region PD. Where the intensity of the incident light is high, charges accumulated at photodiode region PD overflow into floating diffusion region 12. The overflowing charges (50) increase the amount of charge accumulated in floating diffusion region 12. As a result, the brightness of a pixel corresponding to the pixel sensor increases as a time interval between when the pixel sensor is exposed and when it is read increases.
The operation of a CIS can be roughly divided into three basic operations: a reset operation for removing electrical charges accumulated in pixel sensors during a previous frame, an exposure operation for accumulating electrical charges in the pixel sensors, and a read operation for sensing potential variation caused by the accumulated electrical charges. Because the CIS is not able to read all of its pixel sensors at the same time, the read operation is performed for each pixel sensor according to a predetermined sequence.
FIG. 2 shows an image captured by a conventional CIS. In FIG. 2, image defects such as white spots caused by an overflow of charges from a photodiode region to a floating diffusion region are clearly visible.
Pixel sensors in the conventional CIS perform read operations in a predetermined sequence as described above. In FIG. 2, the read operations were performed in a raster scan order beginning at a top corner of the image. However, the pixel sensors perform the exposure operation at roughly the same time. Accordingly, a time lapse between the exposure operation and the read operation increases from the top of the image to the bottom.
Because the time lapse between the exposure operation and the read operation increases toward the bottom of the image, the intensity of pixels toward the bottom of the image tends to be brighter than the intensity of pixels at the top of the image. This is due to the previously discussed overflow of charges from the respective photodiode regions to the floating diffusion regions of the pixel sensors in the CIS.
In order to minimize image distortion caused by charges overflowing into the floating diffusion region, U.S. Pat. No. 5,986,297 proposes a method by which charges are accumulated in a metal oxide semiconductor (MOS) capacitor rather than floating diffusion region 12. Unfortunately, however, the MOS capacitor does not entirely prevent the overflow of charges from the photodiode region.